Role of electron-lattice couplings on charge order in quasi-two-dimensional organic conductors

Charge ordering with structural distortion in quasi-two-dimensional organic conductors θ-(ET)2RbZn(SCN)4 (ET=BEDT-TTF) and α-(ET)2I3 is investigated for an extended Hubbard model with Peierls-type electron-lattice interactions within the Hartree-Fock approximation. It is found that the lattice effects stabilize the experimentally observed charge order substantially in θ-(ET)2RbZn(SCN)4, whereas the energy gain by a lattice distortion is small and its band structure and Coulomb interactions play an important role to realize the charge order in α-(ET)2I3. The results are consistent with experimental observations and show contrastive roles of lattice degrees of freedom on charge orders in these compounds.